Unlike that of a single hop wireless network, the wireless communication channel of a multi-hop wireless network is a shared communication channel. In the multi-hop wireless network, only nodes within a given coverage area of a central node (referred to as neighbor nodes) can receive packet information sent by the central node, whereas nodes outside of the coverage area of the central node cannot sense the existence of packet information being communicated between the central node and its neighbor nodes. This is one of the advantages of the multi-hop wireless network in that the nodes outside of the given coverage area of a receiving node (also known as the central node) will not be affected by the data transmission and the packet information exchange within the coverage area. Therefore, the nodes outside of the coverage area of the receiving node can transmit their own packet information and data to their respective neighbor nodes at the same time.
In the conventional art, one of the problems with multi-hop wireless communication is that the location of the nodes within the given coverage area greatly affects the chance of a data collision occurred in the area. In the communication channel of the single hop wireless network, data collision is a global event. In other words, if not all of the nodes receive the correct packet information, all nodes will sense the data collision. However, because the multi-hop wireless network utilizes the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) method, data collision is a local event. That is, nodes that are not within the coverage area of the central node may not receive the packet information and may consequently become “hidden nodes” to the central node. These hidden nodes may be problematic as they move within the coverage area of the central node because they have not received any packet information about the central node and because the central node does not recognize them as its neighbor node. As a result, the hidden nodes may mistakenly send data to the central node when the central node is busy transmitting data with one of its neighbor nodes, causing data collision in the network.
Refer to FIG. 1, which illustrates an example of a coverage area of each node in the wireless communication network in accordance with one embodiment of the present invention. As shown in FIG. 1, when node C sends out its packet information, because node A is not within the coverage area of node C, node A becomes a hidden node that cannot receive the packet information sent by node C. As a result, node A may mistakenly send data to node C when node C is transmitting data with one of its neighbor nodes, such as node D, causing data collision within the coverage area of node C.
One way to resolve the hidden node problem in the conventional art is to for a transmitting node to perform a short handshake with a receiving node before the transmitting node begins to transmit data. For example, sending a Request to Send (RTS) signal and/or a Clear to Send (CTS) signal is one way to implement the short handshake.
However, the introduction of sending the RTS and CTS signals will affect the overall network performance. If a transmission power of the CTS signal is too large, then irrelevant nodes in the network will be mistakenly notified. This will causes the irrelevant nodes to “wait” unnecessarily, thereby reducing the network performance. On the other hand, if the transmission power of the CTS signal is too small, then not all neighbor nodes may be notified. As a result, data collision may occur, thereby reducing the overall network performance as well. Therefore, what is needed is a method and system capable of reducing noise and expanding bandwidth capabilities in the multi-hop wireless network.